1. Field of the Invention
This invention relates to the manufacture of integrated circuit modules and of circuit boards including integrated circuit modules, and, more particularly, to the attachment of heat sinks to integrated circuit modules within circuit board assemblies.
2. Summary of the Background Art
Due to decreases in the size of circuit elements, such as transistors within integrated circuit modules, and additionally due to increases in the clock frequencies used within computing systems, integrated circuits are being built in smaller sizes with increased levels of capability, often resulting in increased levels of heat generation. This statement is particularly true for the chips supporting controllers in personal computer. Such support chips are now being designed to dissipate significantly more power than similar devices from previous generations of circuits. Additionally, more powerful support chips are being designed without integrated heat spreaders (IHSs), which have conventionally been used to achieve effective cooling.
One traditional method for cooling a large circuit chip, such as an ASIC having a suitably large surface area has been to fasten a heat sink to the surface with epoxy. However, as the circuit chips grow smaller, and as the heat sinks required to dissipate increasing levels of power grow larger, the fragility of the silicon structure to which the heat sink is bonded becomes a significant problem. In particular, a massive heat sink can cause cracking of the silicon due to mechanical shock and vibration. Thus, what is needed is a method providing for the efficient transfer of heat from a circuit chip without requiring the attachment of a heat sink directly to the circuit chip.
Another traditional method, which has been used extensively to provide cooling for circuit chips, is to hold a heat sink against the surface of a circuit chip by means of an external spring clip. However, the use of an external spring clip typically requires the existence of significant clip mounting space outside the footprint of the circuit chip. Such space is becoming increasingly difficult to provide, as it is becoming necessary to provide increased levels of function in smaller sections of a circuit board.
A method for cooling a very small integrated circuit package, and for mounting the package on a circuit board, is described in U.S. Pat. No. 5,359,768. According to this method, the circuit package includes a circuit chip or die that is attached to the heat sink using an adhesive before the die is attached to the circuit board. The die includes solder bumps facing the circuit board for direct attachment to contacts on the surface of the circuit board. Since the heat sink is attached to the die before the die is attached to the circuit board, the heat sink may be used to facilitate handling the smaller substrate during the manufacturing and testing processes. In one embodiment, the solder connection between the circuit board and the die is made by heating the chip to reflow the solder. In another embodiment, the connection is made by using a shrinking adhesive placed to extend around the die and between the heat sink and the circuit board. Again, what is needed is a method providing for the efficient transfer of heat from the circuit chip or die without attaching a heat sink directly to the die, so that damage to the silicon material of the die due to shock and vibration can be prevented.
U.S. Pat. No. 6,409,859 describes an electronic package or module protected by a lid sealed with a thermoplastic or thermosetting laminate that has been previously applied onto the lid, including the bonding area of the lid. The adhesive is deposited as wet adhesive or is laminated in sheet format onto a sheet of the material of which the lids are to be formed. The adhesive is dried or B-staged to become a solid sheet perform, as by solvent removal or chemical cross-linking, respectively. Again, what is needed is a method providing for efficient heat transfer without the attachment of a heat dissipating structure directly to the circuit chips.
U.S. Pat. No. 4,005,457 describes a semiconductor assembly devoid of wire bonds, comprising a semiconductor, an electrode-containing substrate supporting the semiconductor, and an electrode containing substrate supported by the semiconductor. A cure-shrinkable adhesive means incorporating an oxide remover (preferably a halogen) physically bonds together the semiconductor and the cover electrode. This assembly method is applied particularly to semiconductor chips forming light emitting diodes.